Many business and home consumers are requesting faster data access than can be provided by voice-band modems over two-wire subscriber lines. The central office switch, because of the voice band CODEC's, limits the data bandwidth to approximately 56 kbps. In response, both incumbent local exchange carriers (LEC's) and alternative carries are offering digital subscriber line (DSL) service. Basically, DSL uses wide frequency band above the voice frequency to carry more data. The same tip-ring pair that supports voice service can thus also support DSL service without rewiring the extant local telephone system.
When DSL service is added over voice service, several filters and other components are added to the subscriber loop in order to isolate the DSL service from the voice service at both the customer premises and at the central office to avoid one interfering with the other. When these components are added, voice band service is affected negatively. Telcordia (formerly BellCore) specified voice-band characteristics in the LSSGR (LATA Switching System Generic Requirements), now Telcordia GR 507 (local switching) and GR57 (DLC). Currently, when a DSL line card is connected to a POTS line card, the POTS line card does not meet these specifications.
Special line cards have been proposed to meet the requirements while still providing DSL services, but this requires that the POTS line card be changed when DSL service is added. Therefore, a challenge in the art is to develop a POTS line card that can meet LSSGR requirements both when it is and is not connected in circuit with a DSL line card.